A printed wiring board has made a great progress in the last half century and today has been used for almost all electronic devices. High-density implementation of mounted components and handling of signals at higher frequencies have been progressed along with the increase of the recent needs for smaller electronic devices with a higher performance, and printed wiring boards are needed to have a fine conductive pattern (fine pitch) and to deal with high frequencies, for example. Particularly in the case that an IC chip is mounted on a printed wiring board, a fine pitch with L (line)/S (space)=20 μm/20 μm or less is required.
A printed wiring board is first fabricated as a copper-clad laminate in which a copper foil is pasted on an insulating substrate primarily containing a glass epoxy substrate, a BT resin, a polyimide film, or the like. For pasting is employed a method (laminating method) in which an insulating substrate and a copper foil are stacked together followed by heating and pressing for formation, or a method (casting method) in which the face having a coating layer of a copper foil is coated with a varnish as a precursor for an insulating substrate material followed by heating/curing.
The foil thickness is getting smaller in association with the popularization of a fine pitch; for example, the thickness of a copper foil used for a copper-clad laminate is now 9 μm or less, or even 5 μm or less. However, the foil thickness of 9 μm or less extremely deteriorates the handleability in forming a copper-clad laminate using the above-described laminating method or casting method. Accordingly, a copper foil provided with a carrier has been developed in which an ultrathin copper layer is formed above a thick metal foil, which is utilized for a carrier, with a peel layer sandwiched therebetween. In a common method for using a copper foil provided with a carrier, the surface of the ultrathin copper layer is pasted on a resin substrate to heat and pressure-bond and thereafter the carrier is peeled off via the peel layer, as disclosed in Patent Literature 1 or the like.
In a typical method for using a copper foil provided with a carrier in producing a printed wiring board using a copper foil provided with a carrier, a copper foil provided with a carrier is first laminated on a resin substrate from the ultrathin copper layer side and then the carrier is peeled from the ultrathin copper layer. Next, a plating resist formed from a photocurable resin is provided on the exposed ultrathin copper layer from which the carrier has been peeled. And then, a predetermined region of the plating resist is exposed to a light to cure the region. Subsequently, the uncured plating resist in the non-exposed region is removed and thereafter an electrolytic plating layer is provided in the resist-removed region. The cured plating resist is then removed to obtain a resin substrate on which a circuit is formed, which is used to produce a printed wiring board.